there are many companies which abuse gnutella and spam it with
malicious or corrupt files. to prevent downloading these files,
you can do following things:
check the file name. if it has nothing to do with the keyword
you entered, don't download it. if you notice that many fake files
come from the same IP, you can block it (i'm not sure if you can
in mactella, but in LimeWire you can).
check the size. if you download a mp3, the size is usually
between 3 mb - 10 mb, that depends on the song length and the
quality.

if it is a movie, it is usually + 100mb

there was also a file called hostile.net which listed all bad clients,
i will look that up for you./
hope this helps.